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NAME |LIBRARY |SYNOPSIS |DESCRIPTION |RETURN VALUE |ERRORS |ATTRIBUTES |VERSIONS |STANDARDS |HISTORY |NOTES |EXAMPLES |SEE ALSO |COLOPHON | |
eventfd(2) System Calls Manualeventfd(2)eventfd - create a file descriptor for event notification
Standard C library (libc,-lc)
#include <sys/eventfd.h>int eventfd(unsigned intinitval, intflags);
eventfd() creates an "eventfd object" that can be used as an event wait/notify mechanism by user-space applications, and by the kernel to notify user-space applications of events. The object contains an unsigned 64-bit integer (uint64_t) counter that is maintained by the kernel. This counter is initialized with the value specified in the argumentinitval. As its return value,eventfd() returns a new file descriptor that can be used to refer to the eventfd object. The following values may be bitwise ORed inflags to change the behavior ofeventfd():EFD_CLOEXEC(since Linux 2.6.27) Set the close-on-exec (FD_CLOEXEC) flag on the new file descriptor. See the description of theO_CLOEXECflag inopen(2) for reasons why this may be useful.EFD_NONBLOCK(since Linux 2.6.27) Set theO_NONBLOCKfile status flag on the open file description (seeopen(2)) referred to by the new file descriptor. Using this flag saves extra calls tofcntl(2) to achieve the same result.EFD_SEMAPHORE(since Linux 2.6.30) Provide semaphore-like semantics for reads from the new file descriptor. See below. Up to Linux 2.6.26, theflags argument is unused, and must be specified as zero. The following operations can be performed on the file descriptor returned byeventfd():read(2) Each successfulread(2) returns an 8-byte integer. Aread(2) fails with the errorEINVALif the size of the supplied buffer is less than 8 bytes. The value returned byread(2) is in host byte order—that is, the native byte order for integers on the host machine. The semantics ofread(2) depend on whether the eventfd counter currently has a nonzero value and whether theEFD_SEMAPHOREflag was specified when creating the eventfd file descriptor: • IfEFD_SEMAPHOREwas not specified and the eventfd counter has a nonzero value, then aread(2) returns 8 bytes containing that value, and the counter's value is reset to zero. • IfEFD_SEMAPHOREwas specified and the eventfd counter has a nonzero value, then aread(2) returns 8 bytes containing the value 1, and the counter's value is decremented by 1. • If the eventfd counter is zero at the time of the call toread(2), then the call either blocks until the counter becomes nonzero (at which time, theread(2) proceeds as described above) or fails with the errorEAGAINif the file descriptor has been made nonblocking.write(2) Awrite(2) call adds the 8-byte integer value supplied in its buffer to the counter. The maximum value that may be stored in the counter is the largest unsigned 64-bit value minus 1 (i.e., 0xfffffffffffffffe). If the addition would cause the counter's value to exceed the maximum, then thewrite(2) either blocks until aread(2) is performed on the file descriptor, or fails with the errorEAGAINif the file descriptor has been made nonblocking. Awrite(2) fails with the errorEINVALif the size of the supplied buffer is less than 8 bytes, or if an attempt is made to write the value 0xffffffffffffffff.poll(2)select(2) (and similar) The returned file descriptor supportspoll(2) (and analogouslyepoll(7)) andselect(2), as follows: • The file descriptor is readable (theselect(2)readfds argument; thepoll(2)POLLINflag) if the counter has a value greater than 0. • The file descriptor is writable (theselect(2)writefds argument; thepoll(2)POLLOUTflag) if it is possible to write a value of at least "1" without blocking. • If an overflow of the counter value was detected, thenselect(2) indicates the file descriptor as being both readable and writable, andpoll(2) returns aPOLLERR event. As noted above,write(2) can never overflow the counter. However an overflow can occur if 2^64 eventfd "signal posts" were performed by the KAIO subsystem (theoretically possible, but practically unlikely). If an overflow has occurred, thenread(2) will return that maximumuint64_t value (i.e., 0xffffffffffffffff). The eventfd file descriptor also supports the other file- descriptor multiplexing APIs:pselect(2) andppoll(2).close(2) When the file descriptor is no longer required it should be closed. When all file descriptors associated with the same eventfd object have been closed, the resources for object are freed by the kernel. A copy of the file descriptor created byeventfd() is inherited by the child produced byfork(2). The duplicate file descriptor is associated with the same eventfd object. File descriptors created byeventfd() are preserved acrossexecve(2), unless the close-on- exec flag has been set.
On success,eventfd() returns a new eventfd file descriptor. On error, -1 is returned anderrno is set to indicate the error.
EINVALAn unsupported value was specified inflags.EMFILEThe per-process limit on the number of open file descriptors has been reached.ENFILEThe system-wide limit on the total number of open files has been reached.ENODEVCould not mount (internal) anonymous inode device.ENOMEMThere was insufficient memory to create a new eventfd file descriptor.
For an explanation of the terms used in this section, seeattributes(7). ┌──────────────────────────────────────┬───────────────┬─────────┐ │Interface│Attribute│Value│ ├──────────────────────────────────────┼───────────────┼─────────┤ │eventfd() │ Thread safety │ MT-Safe │ └──────────────────────────────────────┴───────────────┴─────────┘
C library/kernel differences There are two underlying Linux system calls:eventfd() and the more recenteventfd2(). The former system call does not implement aflags argument. The latter system call implements theflags values described above. The glibc wrapper function will useeventfd2() where it is available.Additional glibc features The GNU C library defines an additional type, and two functions that attempt to abstract some of the details of reading and writing on an eventfd file descriptor: typedef uint64_t eventfd_t; int eventfd_read(int fd, eventfd_t *value); int eventfd_write(int fd, eventfd_t value); The functions perform the read and write operations on an eventfd file descriptor, returning 0 if the correct number of bytes was transferred, or -1 otherwise.
Linux, GNU.
eventfd() Linux 2.6.22, glibc 2.8.eventfd2() Linux 2.6.27 (see VERSIONS). Since glibc 2.9, theeventfd() wrapper will employ theeventfd2() system call, if it is supported by the kernel.
Applications can use an eventfd file descriptor instead of a pipe (seepipe(2)) in all cases where a pipe is used simply to signal events. The kernel overhead of an eventfd file descriptor is much lower than that of a pipe, and only one file descriptor is required (versus the two required for a pipe). When used in the kernel, an eventfd file descriptor can provide a bridge from kernel to user space, allowing, for example, functionalities like KAIO (kernel AIO) to signal to a file descriptor that some operation is complete. A key point about an eventfd file descriptor is that it can be monitored just like any other file descriptor usingselect(2),poll(2), orepoll(7). This means that an application can simultaneously monitor the readiness of "traditional" files and the readiness of other kernel mechanisms that support the eventfd interface. (Without theeventfd() interface, these mechanisms could not be multiplexed viaselect(2),poll(2), orepoll(7).) The current value of an eventfd counter can be viewed via the entry for the corresponding file descriptor in the process's/proc/pid/fdinfo directory. Seeproc(5) for further details.
The following program creates an eventfd file descriptor and then forks to create a child process. While the parent briefly sleeps, the child writes each of the integers supplied in the program's command-line arguments to the eventfd file descriptor. When the parent has finished sleeping, it reads from the eventfd file descriptor. The following shell session shows a sample run of the program: $./a.out 1 2 4 7 14 Child writing 1 to efd Child writing 2 to efd Child writing 4 to efd Child writing 7 to efd Child writing 14 to efd Child completed write loop Parent about to read Parent read 28 (0x1c) from efdProgram source #include <err.h> #include <inttypes.h> #include <stdio.h> #include <stdlib.h> #include <sys/eventfd.h> #include <sys/types.h> #include <unistd.h> int main(int argc, char *argv[]) { int efd; uint64_t u; ssize_t s; if (argc < 2) { fprintf(stderr, "Usage: %s <num>...\n", argv[0]); exit(EXIT_FAILURE); } efd = eventfd(0, 0); if (efd == -1) err(EXIT_FAILURE, "eventfd"); switch (fork()) { case 0: for (size_t j = 1; j < argc; j++) { printf("Child writing %s to efd\n", argv[j]); u = strtoull(argv[j], NULL, 0); /* strtoull() allows various bases */ s = write(efd, &u, sizeof(uint64_t)); if (s != sizeof(uint64_t)) err(EXIT_FAILURE, "write"); } printf("Child completed write loop\n"); exit(EXIT_SUCCESS); default: sleep(2); printf("Parent about to read\n"); s = read(efd, &u, sizeof(uint64_t)); if (s != sizeof(uint64_t)) err(EXIT_FAILURE, "read"); printf("Parent read %"PRIu64" (%#"PRIx64") from efd\n", u, u); exit(EXIT_SUCCESS); case -1: err(EXIT_FAILURE, "fork"); } }futex(2),pipe(2),poll(2),read(2),select(2),signalfd(2),timerfd_create(2),write(2),epoll(7),sem_overview(7)
This page is part of theman-pages (Linux kernel and C library user-space interface documentation) project. Information about the project can be found at ⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report for this manual page, see ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩. This page was obtained from the tarball man-pages-6.15.tar.gz fetched from ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on 2025-08-11. If you discover any rendering problems in this HTML version of the page, or you believe there is a better or more up- to-date source for the page, or you have corrections or improvements to the information in this COLOPHON (which isnot part of the original manual page), send a mail to man-pages@man7.orgLinux man-pages 6.15 2025-05-17eventfd(2)Pages that refer to this page:io_uring_register(2), signalfd(2), syscalls(2), timerfd_create(2), io_uring_register_eventfd(3), io_uring_register_eventfd_async(3), io_uring_unregister_eventfd(3), proc_pid_fd(5), proc_pid_fdinfo(5), systemd.exec(5)
HTML rendering created 2025-09-06 byMichael Kerrisk, author ofThe Linux Programming Interface. For details of in-depthLinux/UNIX system programming training courses that I teach, lookhere. Hosting byjambit GmbH. | |